A fuel cell comprising a polymer electrolyte membrane generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions and a pair of electrodes formed on both surfaces of the polymer electrolyte membrane, i.e., an anode and a cathode. The electrode usually comprises a catalyst layer which is composed mainly of carbon particles carrying a platinum group metal catalyst and is formed on the surface of the polymer electrolyte membrane and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
In order to prevent the fuel gas and oxidant gas supplied to the electrodes from leaking out or prevent these two kinds of gases from mixing together, gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane therebetween. The gaskets are normally composed of a rubber or an elastomer having high chemical resistance such as EPDM rubber, silicone elastomer and fluoro-elasomer. The gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called “MEA” (electrolyte membrane-electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series, or in some cases, in parallel. The separator plates have a gas flow channel for supplying a reaction gas to the electrode surface and for removing a generated gas and an excess gas at a portion to come in contact with the MEA. Although the gas flow channel may be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow channel.
In order to supply the fuel gas and oxidant gas to such grooves, it is necessary to use piping jigs which branch respective supply pipes for fuel gas and oxidant gas, depending on the number of the separator plates to be used, and connect the branches directly to the grooves of the separator plates. This jig is called “manifold”, and the above-described type, directly connecting the supply pipes for fuel gas and oxidant gas with the grooves, is called “external manifold”. A manifold having a simpler structure is called “internal manifold”. In the internal manifold, the separator plates with the gas flow channel formed thereon are provided with through holes which are connected to the inlet and outlet of the gas flow channel such that the fuel gas and oxidant gas are supplied directly from these holes.
Since the fuel cell generates heat during operation, it needs cooling with cooling water or the like to keep the cell under good temperature conditions. Normally, a cooling section for flowing the cooling water therein is formed every one to three cells. The cooling section is inserted between the separator plates in one structure and the cooling section is formed by providing the backsides of the separator plates with a cooling water flow channel in the other structure, and the latter is often employed. In a general structure of the fuel cell, the MEAs, separator plates and cooling sections are alternately stacked to form a stack of 10 to 200 cells, and the resultant stack is sandwiched by end plates with current collector plates and insulating plates and is clamped with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separator plates need to have a high conductivity, high gas tightness with respect to the fuel gas, and high corrosion resistance to oxidation/reduction reactions of hydrogen/oxygen. For such reasons, conventional separator plates have usually been formed from carbon materials such as glassy carbon and expanded graphite, and the gas flow channel has been formed by cutting the surface thereof or by molding in the case of expanded graphite. In recent years, however, an attempt is made to use a metal plate such as stainless steel in place of the conventionally used carbon materials.
The conventional method of cutting the carbon plate has a difficulty in reducing not only the material cost of the carbon plate but also the cutting cost, while the method of using the expanded graphite is also costly with respect to the material, which is considered to be a hindrance to practical use.
The method using the metal plate proposes to produce a separator plate by press working in order to reduce the cost. However, it has a problem that the cost merit is consequently impaired by the limitation of the pattern of the gas flow channel worked on the separator plate and the necessity to use post-treatment for removing the distortion caused by the press and a special material having a high extensibility.